Doctoral Thesis: Massive MIMO in Real Propagation Environments

Mobile communication systems are now evolving towards the fifth generation (5G). In the near future, we expect an explosive increase in the number of connected devices, such as phones, tablets, wearable accessories, sensors, connected vehicles and so on. We can imagine that much higher data rates than in today's 4G systems are required. Sounds like the future of communication is specially developed for urban living? The answer is not really! The 5G visions also include bringing the current "4 billion unconnected" population in remote regions into the Internet world. There is also a great interest in "green communications", aiming for less energy consumption and carbon emissions from the ICT... (More)

Popular Abstract in English

Mobile communication systems are now evolving towards the fifth generation (5G). In the near future, we expect an explosive increase in the number of connected devices, such as phones, tablets, wearable accessories, sensors, connected vehicles and so on. We can imagine that much higher data rates than in today's 4G systems are required. Sounds like the future of communication is specially developed for urban living? The answer is not really! The 5G visions also include bringing the current "4 billion unconnected" population in remote regions into the Internet world. There is also a great interest in "green communications", aiming for less energy consumption and carbon emissions from the ICT (information and communication technology) industry.

One of the technologies that has a potential to fulfill these requirements and visions is massive MIMO (massive multiple-input multiple output). Massive MIMO is a relatively new research area that starts around five years ago. Literally, massive MIMO means that a large number of antennas and transceivers are used on the base-station side in a communication system. By having more antennas, we can basically exploit more degrees of freedom in the spatial domain. With a massive number of antennas, we open up the spatial dimension on a much larger scale than ever before. Through spatial signal processing, we gain higher data rates and communicate with more connected devices simultaneously. Thanks to the large spatial degrees of freedom, the communication service can also cover a larger area, which is especially preferable in remote regions. On the other hand, we can also reduce the transmit power radiated from base stations and terminals. This makes massive MIMO a candidate also in the area of "green communications".

A lot of theoretical analysis has been done, showing the extraordinary advantages of massive MIMO, both in terms of spectral efficiency and transmit-power efficiency. Despite all these advantages, we face many challenges when bringing massive MIMO from theory to practice. The challenges include 1) whether physical propagation environments give the same advantages as predicted in theoretical studies, and 2) how can system complexity stay low at the same time as the number of transceivers becomes "massive". The PhD thesis covers the concepts of massive MIMO, its pros and cons, and tries to answer the above two questions. Based on conducted channel measurements at Lund University, we focus on massive MIMO propagation behavior, modeling and performance evaluation in real-life environments. The "ideal" world of theory has been connected to the "non-ideal" reality. Not much has been known about massive MIMO behavior in real propagation environments, and whether the claims about massive MIMO also hold there, until the studies in this thesis were done. (Less)

Abstract

Mobile communications are now evolving towards the fifth generation (5G). In the near future, we expect an explosive increase in the number of connected devices, such as phones, tablets, sensors, connected vehicles and so on. Much higher data rates than in today's 4G systems are required. In the 5G visions, better coverage in remote regions is also included, aiming for bringing the current "4 billion unconnected" population into the online world. There is also a great interest in "green communications", for less energy consumption in the ICT (information and communication technology) industry.

Massive MIMO is a potential technology to fulfill the requirements and visions. By equipping a base station with a large number,... (More)

Mobile communications are now evolving towards the fifth generation (5G). In the near future, we expect an explosive increase in the number of connected devices, such as phones, tablets, sensors, connected vehicles and so on. Much higher data rates than in today's 4G systems are required. In the 5G visions, better coverage in remote regions is also included, aiming for bringing the current "4 billion unconnected" population into the online world. There is also a great interest in "green communications", for less energy consumption in the ICT (information and communication technology) industry.

Massive MIMO is a potential technology to fulfill the requirements and visions. By equipping a base station with a large number, say tens to hundreds, of antennas, many terminals can be served in the same time-frequency resource without severe inter-user interference. Through "aggressive" spatial multiplexing, higher data rates can be achieved without increasing the required spectrum. Processing efforts can be made at the base station side, allowing terminals to have simple and cheap hardware. By exploiting the many spatial degrees of freedom, linear precoding/detection schemes can be used to achieve near-optimal performance. The large number of antennas also brings the advantage of large array gain, resulting in an increase in received signal strength. Better coverage is thus achieved. On the other hand, transmit power from base stations and terminals can be scaled down to pursue energy efficiency.

In the last five years, a lot of theoretical studies have been done, showing the extraordinary advantages of massive MIMO. However, the investigations are mainly based on theoretical channels with independent and identically distributed (i.i.d.) Gaussian coefficients, and sometimes assuming unlimited number of antennas. When bringing this new technology from theory to practice, it is important to understand massive MIMO behavior in real propagation channels using practical antenna arrays. Not much has been known about real massive MIMO channels, and whether the claims about massive MIMO still hold there, until the studies in this thesis were done.

The thesis study connects the "ideal" world of theory to the "non-ideal" reality. Channel measurements for massive MIMO in the 2.6 GHz band were performed, in different propagation environments and using different types of antenna arrays. Based on obtained real-life channel data, the studies include

The investigations in the thesis conclude that massive MIMO works efficiently in real propagation environments. The theoretical advantages, as observed in i.i.d. Rayleigh channels, can also be harvested in real channels. Important propagation effects are identified for massive MIMO scenarios, including channel variations over large arrays, multipath-component (MPC) lifetime, and 3D propagation. These propagation properties are modeled and included into the COST 2100 MIMO channel model as an extension for massive MIMO. The study on antenna selection shows that characteristics in real channels allow for significant reductions of massive MIMO complexity without significant performance loss.

As one of the world's first research work on massive MIMO behavior in real propagation channels, the studies in this thesis promote massive MIMO as a practical technology for future communication systems. (Less)

@phdthesis{92a24350-c63d-4037-91e0-2f010ba20cb6,
abstract = {Mobile communications are now evolving towards the fifth generation (5G). In the near future, we expect an explosive increase in the number of connected devices, such as phones, tablets, sensors, connected vehicles and so on. Much higher data rates than in today's 4G systems are required. In the 5G visions, better coverage in remote regions is also included, aiming for bringing the current "4 billion unconnected" population into the online world. There is also a great interest in "green communications", for less energy consumption in the ICT (information and communication technology) industry.<br/><br>
<br/><br>
Massive MIMO is a potential technology to fulfill the requirements and visions. By equipping a base station with a large number, say tens to hundreds, of antennas, many terminals can be served in the same time-frequency resource without severe inter-user interference. Through "aggressive" spatial multiplexing, higher data rates can be achieved without increasing the required spectrum. Processing efforts can be made at the base station side, allowing terminals to have simple and cheap hardware. By exploiting the many spatial degrees of freedom, linear precoding/detection schemes can be used to achieve near-optimal performance. The large number of antennas also brings the advantage of large array gain, resulting in an increase in received signal strength. Better coverage is thus achieved. On the other hand, transmit power from base stations and terminals can be scaled down to pursue energy efficiency.<br/><br>
<br/><br>
In the last five years, a lot of theoretical studies have been done, showing the extraordinary advantages of massive MIMO. However, the investigations are mainly based on theoretical channels with independent and identically distributed (i.i.d.) Gaussian coefficients, and sometimes assuming unlimited number of antennas. When bringing this new technology from theory to practice, it is important to understand massive MIMO behavior in real propagation channels using practical antenna arrays. Not much has been known about real massive MIMO channels, and whether the claims about massive MIMO still hold there, until the studies in this thesis were done.<br/><br>
<br/><br>
The thesis study connects the "ideal" world of theory to the "non-ideal" reality. Channel measurements for massive MIMO in the 2.6 GHz band were performed, in different propagation environments and using different types of antenna arrays. Based on obtained real-life channel data, the studies include<br/><br>
• channel characterization to identify important massive MIMO properties,<br/><br>
• evaluation of propagation conditions in real channels and corresponding massive MIMO performance,<br/><br>
• channel modeling for massive MIMO to capture the identified channel properties, and<br/><br>
• reduction of massive MIMO hardware complexity through antenna selection.<br/><br>
<br/><br>
The investigations in the thesis conclude that massive MIMO works efficiently in real propagation environments. The theoretical advantages, as observed in i.i.d. Rayleigh channels, can also be harvested in real channels. Important propagation effects are identified for massive MIMO scenarios, including channel variations over large arrays, multipath-component (MPC) lifetime, and 3D propagation. These propagation properties are modeled and included into the COST 2100 MIMO channel model as an extension for massive MIMO. The study on antenna selection shows that characteristics in real channels allow for significant reductions of massive MIMO complexity without significant performance loss.<br/><br>
<br/><br>
As one of the world's first research work on massive MIMO behavior in real propagation channels, the studies in this thesis promote massive MIMO as a practical technology for future communication systems.},
author = {Gao, Xiang},
isbn = { 978-91-7623-647-5},
issn = {1654-790X},
keyword = {Massive MIMO,very-large MIMO,multi-user MIMO,channel measurements,channel modeling,5G,sum-rate capacity,antenna selection,signal processing},
language = {eng},
month = {01},
pages = {270},
publisher = {Lund University},
school = {Lund University},
series = {Series of Licentiate and Doctoral Theses},
title = {Doctoral Thesis: Massive MIMO in Real Propagation Environments},
year = {2016},
}